A number of audio circuits have heretofore been used in digital audio disk reproducing devices or audio amplifiers, some of which are well known as those are named specifically. In terms of frequency characteristics, such audio circuits have been designed under a presumption that a frequency band audible by human ears is in a range from about 20 Hz to 20 kHz. There are some audio circuits which cover frequencies higher than 20 kHz, however, those circuits are designed so that the frequency response curve becomes substantially flat for frequencies higher than 20 kHz similar to the curve in frequencies below 20 kHz.
According to research performed by the present inventors, it has been found that the audible frequency band of the human ears generally extends to frequencies much higher than 20 kHz. Moreover, depending upon how the audio frequencies higher than 20 kHz are electrically dealt with, the reproduced sounds perceived by the human ears are differentiated. For example, if frequencies higher than 20 kHz are properly reproduced, the local position of a musical sound image is stabilized, or the articulation of spoken words is improved.